ICMCTF2010 Session E5: Nano- and Microtribology

Thin film coatings are utilized extensively to modify or functionalize solid surfaces in various technologies. In tribological applications, coatings are typically used to control friction and minimize surface damage due to contact against a counter surface. Particularly in ultra-precision systems such as micro-electro-mechanical-systems (MEMS) and in applications where extremely smooth surfaces are used such as in hard disk drives (HDD), the surface forces that exist at the contact interface of two components can cause significant problems in operation of the device. Thus, it is essential to apply surface coatings to minimize the surface forces. Since these systems require tight tolerances coatings with thickness in the nano-scale range such as self-assembled-monolayer (SAM) and perfluoropolyether (PFPE) are utilized. These coatings, with a thickness in the range of a few nanometers, have been effectively implemented in reducing stiction of MEMS components and head sliders of HDD.

In this presentation an overview of the tribological behavior and characteristics of such ultra-thin films for microsystems applications will be provided. Specifically, the effectiveness of SAM and PFPE based lubricant coatings on wear reduction of silicon based materials will be assessed. Another application where tribological behavior of SAM coating needs to be clearly understood is in probe based lithography where SAM coating is used as a resist layer. The SAM resist layer is preferentially removed by the probe tip to fabricate patterns on the substrate surface. In addition to the general tribological behavior of SAM coating, the physical interaction between SAM coating and a probe tip in probe based lithography will be presented.

Acknowledgement: This work was supported by Ministry of Knowledge Economy (2008-N-PV08-P-06-0-000).

There is an increasing demand for understanding nanostructured coatings properties not only at room temperature but also at high temperature near real application condictions. Recent developments in high-temperature nanoindentation¹ have shown increased interest amongst the research community as a step to get closer to real problems or modelling. Measurement methods have been developed for evaluating the hardness and modulus of Physical Vapour Deposited (magnetron sputtering and cathodic arc) coatings when the temperature is raised from room temperature to 400 degrees. Additional nano-testing capabilities have been used to characterise nano-structured properties, this includes nano-scratch, nano-impact, nano-wear. Atomic Force Microscopy and Scanning Probe Microscopy have been used as a mean of surface analysis. Additional techniques such as Scanning Electron Microscopy, Energy Dispersive Spectroscopy and X-Ray Diffraction techniques have been used to complement the analysis. Physical parameters including temperature or loading curve, coatings deposition technique, coating composition and microstructure have been investigated and a comprehensive database has been produced and some results are reported in this paper.

[1] B. Beake, J.F. Smith, Philosophical Magazine A, Volume 82, Number 10, 1 July 2002 , pp. 2179-2186(8).

Gold is a noble material that has been known for its excellent corrosion resistance, great electrical conductivity, and thermal properties. Therefore, gold is widely used as an electrical conductor in microcomponents/microswitches. Common failure mechanisms, however, when using Au in these components are adhesion, melting, and increase in electrical resistivity due to wear of the coating. Typical strategies to overcome these issues are solid-solution alloying to increase strength and modification of operation conditions of the device. A potential alternative to these strategies has been explored by examining the mechanical properties and microtribology of Au thin films as nanocomposites with small additions of MoS₂ and bilayers with a sacrificial MoS₂ top layer. Results in the literature for macroscopic tribocontacts show that the co-sputtering of MoS₂ with Au increases the wear resistance and the endurance life, but also increases the electrical resistivity significantly when the at% Au falls below roughly 80%. For this study, microtribological properties of bilayered Au/MoS₂ lubricants, with varying thickness of the MoS₂ layer on top of Au, were studied with the purpose to improve the wear resistance. Reciprocating microscratch tests were performed using a nanoindentation instrument with varying the tip radius, contact pressures, and sliding cycles. Single scratch tests with increasing normal load were also performed in order to investigate the adhesion between the different layers. Characterization of the wear track was performed using a micro-Raman spectrometer and an atomic force microscope. The results showed that the nanocomposite Au-MoS₂ coatings had increased the wear resistance compared to pure Au films. Similarly for bilayer Au/MoS₂ samples, the wear resistance of the Au film was enhanced by the MoS₂ top layer, which behaved as a sacrificial layer protecting the Au. The mechanism for this wear resistance was explored by ex situ examination of MoS₂ transfer films and wear debris at specific sliding cycles. Future work will focus on the effect that these modification to the electrode material and structure have on the electrical properties at similar length scales to a MEMS contact.

Keywords: MoS₂, Au, Microtribology, MEMS, microswitches

A new nanotribological testing technique is introduced. It fills a key measurement gap in terms of pressures, forces, contact areas and sliding speed between AFM based wear testing and micro-/macrotribometers and hence more directly maps onto contact conditions in a range of applications including MEMS devices, automotive engines and biomedical devices.

In situ (during the test) measurements of both true nano-scale fretting wear (nano-fretting) or nano-/microscale reciprocating wear are possible over several hours (up to 1000000 wear cycles or more).

The large number of cycles possible in the accelerated nano-wear test - compared to existing nanotribology tools - is due to the inherent stability of the instrumentation (typical signal drift <0.005 nm/s; NanoTest platform, Micro Materials Ltd). The in situ monitoring of nano-wear over such long periods enables tests to be usefully run for greater duration and at lower contact pressures than is typical in previous nanotribological tests. Results are presented on a wide range of hard and soft thin films and are used to optimise coatings for enhanced durability.